This invention relates to the redistribution of organic substituents of halides of silicon to produce more multiorganic substituted silanes. More specifically, this invention relates to a process in which an alkylation step is coupled with a redistribution step to facilitate conversion of halosilanes and organohalosilanes, for example SiCl.sub.4 and CH.sub.3 SiCl.sub.3, to more economically valuable multi-organic substituted species such as (CH.sub.3).sub.2 SiCl.sub.2, (CH.sub.3).sub.3 SiCl and (CH.sub.3).sub.4 Si.
In the silicones industry the multi-organic substituted species such as (CH.sub.3).sub.2 SiCl.sub.2 and (CH.sub.3).sub.3 SiCl are often in high demand and short supply. Diorganodihalosilanes are hydrolyzed to produce diorganopolysiloxane polymers. These polymers find numerous applications as fluids and formulation components of silicone gels, elastomers and resins. Triorganohalosilanes are in demand as end-blockers and as silylating agents.
Organohalosilanes are produced primarily by a process of reacting silicon directly with organic halides, as first disclosed by Rochow and his co-workers in the 1940's. This direct process can be controlled so that the predominant component is the diorganodihalosilane. However, other products of lesser commercial utility are also produced. These other products include tetrahalosilanes and organotrihalosilanes. It would be advantageous if such highly halogenated species could be selectively and efficiently converted to the more useful diorganodihalosilanes, triorganohalosilanes, and tetraorganosilanes.
Methylation of an organohalosilane such as methyltrichlorosilane can be facilitated by a halide-accepting metal such as aluminum. This process is believed to encompass the following steps: EQU MeSiCl.sub.3 +MeCl+2/3Al.fwdarw.Me.sub.2 SiCl.sub.2 +2/3AlCl.sub.3( 1) EQU Me.sub.2 SiCl.sub.2 +MeCl+2/3Al.fwdarw.Me.sub.3 SiCl+2/3AlCl.sub.3( 2) EQU Me.sub.3 SiCl+MeCl+2/3Al.fwdarw.Me.sub.4 Si+2/3AlCl.sub.3 ( 3)
Reaction 1 is considerably slower than reactions 2 and 3 and limits the rate of conversion of methyltrichlorosilane to multi-organic substituted silanes.
Methyltrichlorosilane can also undergo a redistribution reaction as illustrated in reaction 4. EQU Me.sub.4 Si+MeSiCl.sub.3 .fwdarw.Me.sub.3 SiCl+Me.sub.2 SiCl.sub.2( 4)
Reaction 4 can be catalyzed by a metal halide such as AlCl.sub.3 to increase the redistribution rate.
Hurd, J. Am. Chem. Soc. (1945), Vol. 67, p. 1545-1548, and Hurd, U.S. 2,403,370, issued July 2, 1946, disclose the alkylation of tetrachlorosilane and various methylchlorosilanes by passing the vapors of these chlorosilanes together with an alkyl halide over finely divided aluminum, zinc, or other reactive metal at elevated temperature, 300.degree. C. to 500.degree. C. Hurd discloses that a reaction occurs under these conditions in which chlorine groups on the chlorosilane are replaced by alkyl groups.
Sleddon, U.S. 3,135,778, issued June 2, 1964, describes a process using aluminum chloride in the presence of a silane cotaining silicon-bonded hydrogen atoms to redistribute a mixture of trimethylchlorosilane and methyltrichlorosilane to dimethyldichlorosilane.
Turetskaya et al., S.U. 1,162,478 A, June 23, 1985, describes a catalyst powder for methylating chlorosilanes. The catalyst powder consisted of aluminum with 0.05-5 weight percent titanium and up to 100 weight percent silicon powder.
Straussberger et al., U.S. 4,155,927, issued May 22, 1979, discloses a process for preparing trimethylchlorosilane which comprises reacting methyldichlorosilane with methyl chloride and metallic aluminum in the presence of a diatomite.
Halm et. al., Co-pending U.S. patent application Ser. No. 07/258,950, filed Oct. 17, 1988, describes a process for preparing multi-alkylated silanes. The process comprises contacting a halide of silicon, with an alkyl halide in the presence of a halogen accepting metal. The process further comprises a catalyst, such as tin, which improves the efficiency of exchange of alkyl groups from the alkyl halide to the halide of silicon.